The invention relates to a method and apparatus for controlling a rock drill in a drilling machine comprising a frame, a percussion piston provided in the frame and moving in the longitudinal direction thereof, a shank placed on an axial extension of the percussion piston, and at least one piston that is provided in the frame movably in the axial direction thereof, the piston being arranged to act on the shank to push it towards the front of the drilling machine due to a pressure medium acting on the rear surface of the piston, whereupon at least during the drilling the pressure of said pressure medium is such that the combined force of all the pistons acting on the shank and pushing it forward exceeds the feed force acting on the drilling machine during the drilling, so that as the shank rests on all the pistons it is situated at its optimum point of impact, in which method the pressure of said pressure medium acting on the shank is measured.
When holes are drilled in rock with a rock drill, the conditions of drilling vary in different ways. Rock consists of layers of rock material with different degrees of hardness, wherefore the properties affecting the drilling, such as impact power and feed, should be adjusted according to the current drilling resistance. Otherwise the drilling is irregular since the drill propagates rapidly in a soft material and slowly in hard rock. This brings about several problems concerning for example the endurance of the drilling apparatus and the controllability of the drilling process. One example of solving these problems relates to adjusting the impact power of the drilling machine by transferring an impact-transmitting shank forward from the optimum point of impact in the longitudinal direction when a lower impact power is to be transmitted from the percussion piston to the shank. The shank is moved by means of hydraulically operated pistons, which support the shank from behind either directly or via a sleeve. When the pressure of a pressure medium acting in a cylinder space situated behind the pistons is changed, it is possible to adjust the length of movement of the pistons and thus the position of the shank- In this manner, it is possible to transmit a desired amount of capacity via the shank to the drill rod, whereas the rest of the impact is dampened by a damping pad provided at the front end of the percussion piston. Such an arrangement is disclosed in Finnish Patent 84,701.
Finnish Patent Application No. 944,839 discloses a known manner of controlling the drilling capacity of a rock drilling apparatus, wherein the aim is to prevent the occurrence of damage to the drill. The reference discloses that when the drilling machine hits an area where the drilling resistance is smaller and the drill thus penetrates more easily into the rock, the drilling is continued normally except that the operation of the percussion apparatus is stopped completely until the material under operation gets harder and the drilling requires percussion again. The apparatus comprises a piston of a return damper, which moves in the direction of the percussion piston with respect to the frame of the drilling machine and which is able to move forward towards the drill bit when the drilling resistance is temporarily smaller. This leads to a decrease in pressure in the chamber behind the piston, If the pressure falls below a predetermined pressure level, a valve stops the supply of pressure medium to the percussion apparatus, whereupon the percussion piston will not deliver any more blows. When the drill again hits again hard rock and the pressure in the chamber behind the piston exceeds a predetermined pressure limit, the connection to the percussion apparatus is opened and the percussion piston begins to deliver impacts again.
However, the aforementioned prior art arrangements have proved to be insufficient for the efficient and accurate control of drilling machines. They only affect the control of the impact force but they do not provide means for adjusting and controlling the drilling in more various manners. They also cause loss of power, which means that hydraulic pumps, pipes and other hydraulic components must be made unnecessarily large.
The purpose of the present invention is to provide a better and more versatile method and apparatus for controlling the operation of a drilling machine than previously.